There are some very good uses for semi auto air, that maybe are not immediately apparent. The discipline that I shoot most of, LSR, is open to semi auto 22LR and those shooters can get a second shot off if their cross hairs are still on the 10 ring. That can't happen with a single shot or a bolt action magazine rifle as you inevitably lose position when cocking. Semi auto air levels the playing field.
There are also timed events where you shoot 5 shots in 10 seconds, and I defy anyone to shoot that with a bolt action airgun and have any chance of a decent score.
Regarding the issue of prohibition, I've posted elsewhere that the FEO - when approached specifically - said that a semi auto air rifle under 12 fpe is not subject to licensing and therefore anyone (not restricted by Section 21 or under age) is allowed to purchase and possess one, and the police are not interested in knowing about it.
That was about two months ago, when the enquiry was raised.
I'm told that the HO regards them as prohibited and it is suggested that the HO should be listened to and by implication the FEO advice disregarded. Personally if I was a FEO I wouldn't take kindly to being told my advice was secondary to that of an internet contributor.